1. Technical Field
This application relates to an electric power generation operation point control circuit device for a photovoltaic cell and, more particularly, to a device that is an electric power generation operation point control circuit device for photovoltaic cells which are connected in series and allows each of the photovoltaic cells to perform electric power generation at its maximum electric power point.
2. Description of Related Art
In general, an electric power generation voltage of a photovoltaic cell (cell) is lower than operation voltages of various machinery and equipment and chargers. Accordingly, in photovoltaic electric power generation systems, a configuration in which a plurality of photovoltaic cells are connected in series (photovoltaic cell module) is adopted as a method for using the photovoltaic cell for operations of such machinery and equipment and charging of such chargers. In the photovoltaic cell module in which the plurality of photovoltaic cells are connected in series, an unevenness in light reception amount and a shadow or the like might arise on some of the cells due to a difference between installation angles of the respective photovoltaic cells, buildings, and the like. Once the light reception amount variation occurs between the cells, the cell with a smaller electric power generation amount becomes resistance (reverse-bias diode), and then the photovoltaic cell module might be subjected to a decline in output.
More specifically, as is well known in this field, the photovoltaic cell has a characteristic that a current changes as an electric power generation voltage increases from 0 V as is exemplified in FIG. 6. An optimal operation point (referred to as a maximum electric power point or an optimum operation point) is present in generated electric power, and the generated electric power is at its maximum magnitude at the optimal operation point. In the case of the photovoltaic cell module in which the plurality of photovoltaic cells are connected in series as described above, the maximum output points of all the photovoltaic cells substantially correspond to each other, and thus it is assumed that a common current at the maximum output point flows through all the photovoltaic cells connected in series. In actuality, however, the light reception amounts of some of the photovoltaic cells in the photovoltaic cell module might decrease as described above and, in this case, only the electric power generation characteristics of the photovoltaic cells subjected to the decline in light reception amount change for the current to decrease with respect to the electric power generation voltage, causing a maximum output point deviation. Then, the same current flows through the photovoltaic cells that have different maximum output points in the circuit configuration in which the photovoltaic cells are connected in series, and the cell with a smaller light reception amount (cell with a smaller electric power generation amount) becomes resistant to the current and no electric power generation is performed by the cell with a smaller light reception amount (cell with a smaller electric power generation amount) in a case where the current is adapted to the maximum output point of the cell with a larger light reception amount. Then, a reduction in photovoltaic cell module output follows (an output loss ensues and an electric power generation output commensurate with the light reception amount of the photovoltaic cell module cannot be obtained).
In this regard, electric power generation operation point control circuit devices that are capable of individually controlling the respective operation points of the photovoltaic cells which are connected in series as is exemplified in FIG. 7A have been proposed in the following three non-patent documents as devices for avoiding the decline in output that is attributable to the variation in light reception amount of the photovoltaic cells.    Toshihisa Shimizu and six others, Solar/Wind Power Energy Lecture Paper, 1996, pages 57 to 60    Toshihisa Shimizu, FB Technical News No. 56, Nov. 1, 2000, pages 22 to 27    Toshihisa Shimizu and three others, “Generation Control Circuit for Photovoltaic Modules” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 3, MAY 2001, pages 293 to 300
The electric power generation operation point control circuit device controls the electric power generation voltage, by using a multi-stage boosting chopper circuit with respect to the circuit configuration in which the plurality of photovoltaic cells are connected in series, such that the currents at the respective maximum electric power points flow through the photovoltaic cells. Then, all the photovoltaic cells can perform electric power generation substantially at the maximum electric power points. According to the electric power generation operation point control circuit device, even the photovoltaic cell with a smaller light reception amount is allowed to be operated at its maximum output point, and thus generated electric power commensurate with the light reception amount of the photovoltaic cell module is obtained. In addition, the photovoltaic cell subjected to the light reception amount decline does not become a reverse-bias diode, and thus the output loss can be reduced.
Japanese Patent Application Publication No. 6-284601 (JP 6-284601 A) discloses an example of a circuit device that allows a charger to be efficiently charged with an electric power generation output of a photovoltaic cell by reducing a loss of a switching element regarding a configuration in which a chopper circuit is used during a photovoltaic cell operation control.
In the case of installation of an electric power generation apparatus or an electric power generation facility based on the photovoltaic cell module that is formed by the photovoltaic cells which are connected in series, photovoltaic cell columns (hereinafter, referred to as a “photovoltaic cell strings”) are configured normally by the plurality of photovoltaic cells connected in series being aligned in a strip shape as schematically drawn in FIG. 8A and the photovoltaic cell module is configured by the photovoltaic cell strings being aligned in a direction perpendicular to a longitudinal direction of the strip-shaped structure. At this time, the photovoltaic cell strings are arranged such that anodes and cathodes of the adjacent photovoltaic cell strings are opposite in direction to each other and the anode in an end portion of each photovoltaic cell string is connected to the cathode in an end portion of the adjacent photovoltaic cell string. In other words, the photovoltaic cells that are connected in series are placed to have a configuration in which the photovoltaic cell columns are folded into a predetermined number, that is, by photovoltaic cell string unit. In addition, in a case where the electric power generation operation point control circuit device illustrated in FIG. 7A is applied, it is difficult to connect in parallel a circuit element for photovoltaic cell electric power generation voltage regulation such as a capacitor and switching means as exemplified in the electric power generation operation point control circuit device to each photovoltaic cell unit. Accordingly, in an actual photovoltaic cell string, it is normally assumed that the unevenness in light reception amount attributable to a sunlight angle or a shadow or the like is within an allowable range (refer to FIG. 8B) and, in the photovoltaic cell string, the photovoltaic cells are connected in series as they are and the circuit elements (hereinafter, referred to as “voltage regulation circuit elements”) are connected in parallel so that the electric power generation voltages of the respective photovoltaic cell strings are regulated.